Redox environmental

This relation is hence likely to assist the molecular engineering of soluble proteins. Furthermore, since disulfide bridges can be formed or dismantled in accord with redox environmental conditions, the relation presented is likely to enable the type of design fine-tuning that may be required for an environmental modulation of the protein function. 

As with acid-base and complexation titrations, redox titrations are not frequently used in modern analytical laboratories. Nevertheless, several important applications continue to find favor in environmental, pharmaceutical, and industrial laboratories. In this section we review the general application of redox titrimetry. We begin, however, with a brief discussion of selecting and characterizing redox titrants, and methods for controlling the analyte s oxidation state. 

Inorganic Analysis Redox titrimetry has been used for the analysis of a wide range of inorganic analytes. Although many of these methods have been replaced by newer methods, a few continue to be listed as standard methods of analysis. In this section we consider the application of redox titrimetry to several important environmental, public health, and industrial analyses. Other examples can be found in the suggested readings listed at the end of this chapter. 

Another important example of redox titrimetry that finds applications in both public health and environmental analyses is the determination of dissolved oxygen. In natural waters the level of dissolved O2 is important for two reasons it is the most readily available oxidant for the biological oxidation of inorganic and organic pollutants and it is necessary for the support of aquatic life. In wastewater treatment plants, the control of dissolved O2 is essential for the aerobic oxidation of waste materials. If the level of dissolved O2 falls below a critical value, aerobic bacteria are replaced by anaerobic bacteria, and the oxidation of organic waste produces undesirable gases such as CH4 and H2S. 

Environmental chemistry, including complete typical and worst-case compositional analyses, pH, redox potential, dissolved gas content ... 

Thus in all corrosion reactions one (or more) of the reaction products will be an oxidised form of the metal, aquo cations (e.g. Fe (aq.), Fe (aq.)), aquo anions (e.g. HFeO aq.), Fe04"(aq.)), or solid compounds (e.g. Fe(OH)2, Fej04, Fe3 04-H2 0, Fe203-H20), while the other reaction product (or products) will be the reduced form of the non-metal. Corrosion may be regarded, therefore, as a heterogeneous redox reaction at a metal/non-metal interface in which the metal is oxidised and the non-metal is reduced. In the interaction of a metal with a specific non-metal (or non-metals) under specific environmental conditions, the chemical nature of the non-metal, the chemical and physical properties of the reaction products, and the environmental conditions (temperature, pressure, velocity, viscosity, etc.) will clearly be important in determining the form, extent and rate of the reaction. 

The effects of concentration, velocity and temperature are complex and it will become evident that these factors can frequently outweigh the thermodynamic and kinetic considerations detailed in Section 1.4. Thus it has been demonstrated in Chapter 1 that an increase in hydrogen ion concentration will raise the redox potential of the aqueous solution with a consequent increase in rate. On the other hand, an increase in the rate of the cathodic process may cause a decrease in rate when the metal shows an active/passive transition. However, in complex environmental situations these considerations do not always apply, particularly when the metals are subjected to certain conditions of high velocity and temperature. 

Studies of ligands which might provide specificity in binding to various oxidation states of plutonium seems a particularly promising area for futher research. If specific ion electrodes could be developed for the other oxidation states, study of redox reactions would be much facilitated. Fast separation schemes which do not change the redox equilibria and function at neutral pH values would be helpful in studies of behavior of tracer levels of plutonium in environmental conditions. A particularly important question in this area is the role of PuOj which has been reported to be the dominant soluble form of plutonium in some studies of natural waters (3,14). 

The Table shows a great spread in Kd-values even at the same location. This is due to the fact that the environmental conditions influence the partition of plutonium species between different valency states and complexes. For the different actinides, it is found that the Kd-values under otherwise identical conditions (e.g. for the uptake of plutonium on geologic materials or in organisms) decrease in the order Pu>Am>U>Np (15). Because neptunium is usually pentavalent, uranium hexavalent and americium trivalent, while plutonium in natural systems is mainly tetravalent, it is clear from the actinide homologue properties that the oxidation state of plutonium will affect the observed Kd-value. The oxidation state of plutonium depends on the redox potential (Eh-value) of the ground water and its content of oxidants or reductants. It is also found that natural ligands like C032- and fulvic acids, which complex plutonium (see next section), also influence the Kd-value. 

While these calculations provide information about the ultimate equilibrium conditions, redox reactions are often slow on human time scales, and sometimes even on geological time scales. Furthermore, the reactions in natural systems are complex and may be catalyzed or inhibited by the solids or trace constituents present. There is a dearth of information on the kinetics of redox reactions in such systems, but it is clear that many chemical species commonly found in environmental samples would not be present if equilibrium were attained. Furthermore, the conditions at equilibrium depend on the concentration of other species in the system, many of which are difficult or impossible to determine analytically. Morgan and Stone (1985) reviewed the kinetics of many environmentally important reactions and pointed out that determination of whether an equilibrium model is appropriate in a given situation depends on the relative time constants of the chemical reactions of interest and the physical processes governing the movement of material through the system. This point is discussed in some detail in Section 15.3.8. In the absence of detailed information with which to evaluate these time constants, chemical analysis for metals in each of their oxidation states, rather than equilibrium calculations, must be conducted to evaluate the current state of a system and the biological or geochemical importance of the metals it contains. 

Table 16-5 The standard free energy of reaction, AG , for the main environmental redox reactions...

We have reported a simple, green, bench top, economical and environmentally benign room temperature synthesis of MSe (M=Cd or Zn) nanoparticles using starch, PVA and PVP as passivating agents. The whole process is a redox reaction with selenium acting as the oxidant and MSe as the reduction product. An entire "green" chemistry was explored in this synthetic procedure and it is reproducible. The optical spectroscopy showed that all the particles are blue shifted from the bulk band gap clearly due to quantum confinement. Starch capped CdSe nanoparticles showed the presence of monodispersed spherical... 

Transition metal oxides represent a prominent class of partial oxidation catalysts [1-3]. Nevertheless, materials belonging to this class are also active in catalytic combustion. Total oxidation processes for environmental protection are mostly carried out industriaUy on the much more expensive noble metal-based catalysts [4]. Total oxidation is directly related to partial oxidation, athough opposes to it. Thus, investigations on the mechanism of catalytic combustion by transition metal oxides can be useful both to avoid it in partial oxidation and to develop new cheaper materials for catalytic combustion processes. However, although some aspects of the selective oxidation mechanisms appear to be rather established, like the involvement of lattice catalyst oxygen (nucleophilic oxygen) in Mars-van Krevelen type redox cycles [5], others are still uncompletely clarified. Even less is known on the mechanism of total oxidation over transition metal oxides [1-4,6]. 

An environmentally friendly synthesis of 1,2-methylenedioxybenzene (MDB) can be efficiently carried out in the gas phase, by feeding pyrocatechol (PYC) and formaldehyde acetals and using a catalyst containing weak acid sites and redox sites. The Ti-silicalite (TS-1) was identified as the most active and selective catalyst, indicating the role of well-dispersed octahedrally-coordinated Ti" ions in comparison with some model catalysts. 

The principal abiotic processes affecting americium in water is the precipitation and complex formation. In natural waters, americium solubility is limited by the formation of hydroxyl-carbonate (AmOHC03) precipitates. Solubility is unaffected by redox condition. Increased solubility at higher temperatures may be relevant in the environment of radionuclide repositories. In environmental waters, americium occurs in the +3 oxidation state oxidation-reduction reactions are not significant (Toran 1994). 

The multi-component systems developed quite recently have allowed the efficient metal-catalyzed stereoselective reactions with synthetic potential [75-77]. Multi-components including a catalyst, a co-reductant, and additives cooperate with each other to construct the catalytic systems for efficient reduction. It is essential that the active catalyst is effectively regenerated by redox interaction with the co-reductant. The selection of the co-reductant is important. The oxidized form of the co-reductant should not interfere with, but assist the reduction reaction or at least, be tolerant under the conditions. Additives, which are considered to contribute to the redox cycle directly, possibly facilitate the electron transfer and liberate the catalyst from the reaction adduct. Co-reductants like Al, Zn, and Mg are used in the catalytic reactions, but from the viewpoint of green chemistry, an electron source should be environmentally harmonious, such as H2. 

Environmental conditions determine in large part the chemical reactions that will occur when waste is injected. For example, precipitation-dissolution reactions are strongly controlled by pH. Thus, iron oxides, which may be dissolved in acidic wastes, may precipitate when injection-zone mixing increases the pH of the waste. Similarly, redox potential (Eh) exerts a strong control on the type of microbiological degradation of wastes. 

Transition metal oxides, rare earth oxides and various metal complexes deposited on their surface are typical phases of DeNO catalysts that lead to redox properties. For each of these phases, complementary tools exist for a proper characterization of the metal coordination number, oxidation state or nuclearity. Among all the techniques such as EPR [80], UV-vis [81] and IR, Raman, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and NMR, recently reviewed [82] for their application in the study of supported molecular metal complexes, Raman and IR spectroscopies are the only ones we will focus on. The major advantages offered by these spectroscopic techniques are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as crystalline nanophases and (2) they are able to collect information under various environmental conditions [83], We will describe their contributions to the study of both the support (oxide) and the deposited phase (metal complex). 

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